1. Field of the Invention
The invention relates generally to skew correcting circuits for differential signals and, in particular, to circuits for high-speed differential communication links that can measure and adaptively compensate for differential signal skew.
2. Description of Related Art
Today's high-speed communication systems often rely on differential signal protocols to enhance noise immunity, reduce transient currents, and increase effective signal amplitude. As depicted in FIG. 1, a conventional differential communication link 10 includes a transmit end 12 having a transmitter 14, a receive end 16 having a receiver 18, and a pair of transmission lines 20A, B extending therebetween. Both the transmitter 14 and receiver 18 utilize differential signal generation and transmission protocols known in the art.
An illustrated differential signal 19 is comprised of two complementary intra-pair signal components, referred to herein as the P-side signal 21A and the N-side signal 21B. The P-side signal 21A is also known as the positive, true, or non-inverted signal, or component, of the differential signal while the N-side signal 21B is also known as the negative, complement, or inverted signal, or component. Each signal 21A, B includes an AC portion, or signal pulse, 22A, B, having a fixed width.
To effectively transmit and reconstruct the differential signal 19, the signal polarity of the P-side and N-side signals 21A, B must be mirrored copies of one another and the relative timing of the two sides must be identical, or in phase. As is known to those in the art, several design, material, manufacturing, and circuit anomalies can unpredictably and/or undesirably alter the relative intra-signal/inter-side phase relationship between the P-side and N-side signals 21A, B as they propagate through the transmission lines 20A, B. This typically results in a sub-optimal signal relationship between the P-side and N-side 26A, B of the received differential signal 25. As measured with a standard eye diagram (widely used for qualitatively analyzing digital transmission signals), these anomalies reduce the effective eye opening in both the voltage and timing margins at the receive end 16 of the high speed communication link 10. This performance-degrading phenomenon is referred to as differential signal skew.
Recent studies have demonstrated differential signal skew to be significant in printed circuit board (PCB) environments due to what is referred to as the weave effect. Substrates for printed circuit boards are often constructed with a glass weave to provide mechanical reinforcement. Because the dielectric constant of glass is typically higher, or at least different, than the surrounding resin, differential traces routed over such a glass weave structure typically exhibit a measurable amount of trace-to-trace variations in characteristic impedance and propagation delay, thus causing differential signal skew.
To compensate for weave effect, a number of design methods for routing traces to reduce the impact on differential signal skew have been presented. Examples include methods taught by U.S. Pat. App. Pub. Nos. 2004/0262036—Printed Circuit Board Trace Routing Method, 2004/0181764—Conductor Trace Design to Reduce Common Mode Cross-Talk and Timing Skew, and 2007/0223205—Shifted Segment Layout for Differential Signal Traces to Mitigate Bundle Weave Effect. These methods are specific to the passive interconnect within the circuit board substrate and therefore are believed to be effective in reducing the amount of differential signal skew specifically attributable to laminate-related weave effects.
Currently, no known computer-based circuit board design tools employ these design methods to prevent differential signal skew caused by the weave effect. Thus, these methods must be manually implemented by a knowledgeable and skilled board designer. Several of these design methods, such as non-orthogonal and disjoint routing techniques, are likely to require additional routing space, increasing the size, complexity, and cost of the circuit board. Even after these methods become automated and common in high speed communication circuit boards, differential signal skew is also caused by other mechanisms including, for example, imbalanced trace routing, vias, connectors, return paths, and active circuitry. Using these methods to minimize differential signal skew caused by the weave effect does not alleviate the skew-causing effects of these other mechanisms, and therefore are necessarily limited in application and value.
Another known design method related to skew compensation of single-ended and differential signals include U.S. Pat. No. 6,335,647—Skew Adjusting Circuit, U.S. Pat. No. 6,937,681, Skew Correction Apparatus, and U.S. Pat. No. 6,374,361—Skew-Insensitive Low Voltage Differential Receiver. The design methods taught by these references include adjusting the delay of a data or clock signal relative to a distinctly different signal such as a reference clock or other data signals in parallel busses. Such methods improve the timing of sampling circuits at the receiving end of a differential signal link by adjusting the skew of the signal in its entirety. These methods do nothing to improve the integrity, enhance the value, or actually adjust the differential signal skew of the P and N sides within differential signals themselves. Accordingly, these references do not provide a method of adaptively compensating for differential signal skew.
Still other design methods, believed to be suitable for their intended purposes, do not provide adequate correction of differential signal skew. For example one design method taught by U.S. Pat. App. Pub. No. 2006/0256880—Automatic Skew Correction for Differential Circuits monitors the P and N sides of a transmitted differential signal reflected back to the transmitter and correspondingly adjusting the relative timing of the transmitter's P and N outputs. Due to losses in PCB interconnect products (i.e., up to 30 dB for a one-way loss and up to 60 dB [1000:1 voltage reduction] for a round-trip loss), the reflected signal strength can be drastically reduced, thus severely impacting the effectiveness of the internal feedback loop. In low-loss environments where the reflected signal strength can be appreciable, this method assumes that the reflected signals are generated solely at the receiving end of the communication link. However, intermediate structures such as solder balls, connectors, and vias, present larger reflected signals back to the transmitter, thus further reducing the effectiveness of this method.
Other methods disclosed in U.S. Pat. No. 6,909,980—Auto Skew Alignment of High-Speed Differential Eye Diagrams and U.S. Pat. App. Pub. No. 2004/0064765—Differential Detector Employing Analog-to-Digital Converter compensate for signal skew in the receiver. However, each of these methods sample the P and N sides independently using analog-to-digital (A/D) converters, thus eliminating the benefits of using differential signals. In addition, these methods compensate for differential signal skew by delaying internal digital samples, either by shifting the digital samples within the processor or by adjusting the time points where the signals are sampled. As such, these methods do nothing to enhance the actual differential signal at the input to the receiver, but simply improve the displayed waveforms.
A yet further method for adjusting skew in a differential signal is taught by U.S. Pat. No. 6,353,340—Input and Output Circuit With Reduced Skew Between Differential Signals. In several disclosed embodiments, when the individual input P and N sides have opposite polarity, the output P and N sides are allowed to toggle. However, when the differential signal skew is sufficient enough that the input P and N sides have the same polarity, the P and N output sides are held at a steady state or high-impedance state until the trailing side toggles states. Another disclosed embodiment discloses the use of delay lines; however, the delay is again determined by the logic state of the individual P and N sides. In all the disclosed embodiments, the differential signal skew adjustment is determined by the logic state of the individual P and N sides of the input signal and not by the difference between the P and N sides. As such, the benefits of differential signal protocols are lost. Also, the differential signal skew compensation is adjusted instantaneously for each and every signal transition, which can severely impact jitter and duty cycle distortion; thus, degrading the resulting differential signal.
A still further method taught by U.S. Pat. App. Pub. No. 2006/0244505—Intra-Pair Differential Skew Compensation Method and Apparatus for High-Speed Cable Data Transmission Systems, attempts to overcome the issues aforementioned associated with the method of U.S. Pat. No. 6,353,340. This method requires an initial training sequence wherein the differential signal skew compensation is adjusted once and, thereafter, the compensation is fixed. Such a method is only valuable to compensate for static causes of differential signal skew such as the PCB weave effect. However, this method cannot compensate for dynamic causes such as power droop or intersymbol interference. Also, using training patterns is not always feasible in communication links, and the compensation adjustments are dependent on the data pattern. Because any training pattern will have different characteristics than actual data streams, compensating based on a training pattern produces sub-optimal results.
A method taught by U.S. Pat. No. 7,085,337—Adaptive Per-Pair Skew Compensation Method for Extended Reach Differential Transmission, continuously adjusts differential signal skew compensation at the receiver. However, as with a method above, the P and N sides are treated as two different entities, instead of as a single lumped differential signal. As described above, this negates the benefits of using differential signals. Also, this method uses a slicer (i.e., an A/D converter) to digitize a version of the incoming data signal, then reconverts the digital data back into an analog waveform to compare the original and reconstructed analog signals and use the difference between these two signals to control the delay block. To be effective, the reconstructed signal needs to be converted to a digital format and then back to the analog domain within a fraction of the signal's edge rate, which severely limits the maximum data rate with which the method can be used. In addition, the described “delay block” does not directly adjust the delay of the incoming analog signal, but instead performs a complex filtering function on the individual P and N sides of the differential signal, which again reduces the benefits of using differential signals.
Therefore, to ensure reliable performance of high-speed differential communication links, what is needed is a circuit and method for compensating for differential-skew that directly considers the received differential signal (i.e., not the P and N sides individually), does not place additional burden on the PCB design/designer, compensates for static and dynamic changes in differential signal skew, and compensates skew caused by many sources.